Method of recovering water-soluble nonferrous metal sulfates from sulfur-bearing ores

ABSTRACT

Ore with a relatively high sulfur content, containing various amounts of ferrous and nonferrous sulfides, and particularly a relatively small amount of copper sulfide, is subjected to a controlled oxidation leach in an aqueous medium at elevated temperatures and pressures, to convert the nonferrous sulfides into water-soluble sulfates with only slight concomitant oxidation of the ferrous sulfides.

O Umted States Patent [15] 3,642,435 Allen et al. 51 Feb. 15, 1972 [54]METHOD OF RECOVERING WATEIQ 3,186,942 6/1965 Benger ..23/1 17 X SOLUBLENONFERROUS METAL 3,529,957 9/1970 Kunda et a1 ..23/l26 X Z EE FROMSULFUR'BEARING FOREIGN PATENTS OR APPLICATIONS 51,485 12/1955 Canada..23/l17 1 Inventors: Eugene Allen, Memes, Calif; 626,607 8/1961 Canada..23/125 Royce S. Gavrias, Skounotissa, Cyprus [731 Assignee: CyprusMines Corporation, Los Angeles,. 'f 'f Stem m Attorney-Hams, Kiech,Russell & Kern 22] Filed: Nov. 10, 1969 [57] ABSTRACT [21] Appl. No.:875,256 Ore with a relatively high sulfur content, containing variousamounts of ferrous and nonferrous sulfides, and particularly a [52 us.c1 ..23/117,23/125,23/126, relatively small amount of pp sulfide, issubjected to s 75/ 15 controlled oxidation leach in an aqueous medium atelevated [51 Int. Cl. ..C0lg 1/10, COlg 3/10 temperatures and pr toconvert h nonferrous lfi e [58] Field of Search 23/1 17, 125, 126; 75/]l 5 into water-soluble sulfates with only slight concomitant oxidationof the ferrous sulfides. 56 f C't 1 Re ed 12 Claims, 2 Drawing FiguresUNITED STATES PATENTS 3,174,849 3/1965 Mackin et al. ..75/115 COPPEREXTRFICTIDAPPERCENT N O b l l l l l //0 I40 I70 200 230 26:0 2:90 320TEMPERHTURE [Fl METHOD OF RECOVERING WATER-SOLUBLE NONFERROUS METALSULFATES FROM SULFUR- BEARING ORES BACKGROUND OF THE INVENTION Suchuseful or desirable nonferrous metals as copper, nickel and cobalt, aswell as cadmium, lead, gold, silver and zinc, frequently occur assulfide ores in nature. Unfortunately, they seldom occur alone or indesirably high concentrations. Usually they occur in combinations witheach other or with arsenides or as sulfur-arsenic minerals, togetherwith diluent metal minerals such as those of iron sulfides and withvarious useless or inert gangue materials. lndustrially, then, such orespresent not only the problems of removing gangue and diluent minerals toraise the grade, but also those of recovering the metal values from theore concentrates and separating these values from each other.

Heretofore, the most important conventional processes of recoveringcopper, nickel, cobalt and zinc values, for exam ple, from their sulfideores have involved many steps. Usually the valuable sulfides in the oremust be freed of gangue and from each other, as by gravity separation,froth flotation, or the like. This may be followed by additionalregrinding to further separate the sulfide minerals from each other byadditional froth flotation steps and/or this may be followed bypyrometallurgical processes by roasting the concentrate to removevolatile oxides and some sulfur, adding silica and calcium carbonate tothe roasted ore, and heating the mixture in a reverberatory furnace toform a sulfide matte." This matte' is then oxidized to metal, as forexample in a special Bessemer-type converter to obtain a crude bullionof mixed metals which is usually cast into cakes or anodes and dissolvedelectrolytically in an acid electrolyte, i.e., sulfuric acid. Furtherseparation and recovery of the metals involves chemical purification ofthe electrolyte and removal and retreatment or purification of thedifferent metals by various methods.

The desirability of a more direct method of extracting nonferrous metalsfrom their ores, concentrates and concentrator products without smeltinghas long been recognized. Leaching followed by chemical precipitationappears to offer one solution and many attempts have been made toachieve this result. in general, these attempts have failed to receivecommercial acceptance for various reasons For example, most metallicsulfides present in the concentrates are not directly soluble in wateror in any other commercially practical solvent, by conventional methods.As another example, the massive sulfide concentrates and/or otherconcentrator products containing economic but lesser contents of coppersulfide values with iron sulfides as the diluent may be valuable incertain locations as sulfur-containing byproducts, and can be sold forits sulfur content. In the latter example, it is desirable to retain theiron sulfide mineral, pyrite, because it can be marketed for theproduction of sulfuric acid.

Selective flotation separations of the. nonferrous metal sulfides valuesfrom the iron sulfides have been advanced as a solution. However,- suchseparations are dependent upon grinding of the material to free themetal sulfides from each other. It frequently occurs that the nonferrousmetal sulfides and the iron sulfide minerals are so intimatelyassociated and very finely dispersed that it is uneconomical' to. grindthe 5 material to the fineness required to free the sulfide mineralsfrom each other for separation by selective flotation processes. Theassociation of the nonferrous metal sulfides and the iron sulfideminerals may be so finely dispersed that the inclusions of one sulfidemineral within another sulfide mineral may be considered as solidsolution."

Recently, processes .have been developed for directly leaching theferrous and nonferrous sulfide ores, ore concentrates, and otherconcentrator metal sulfide products to dissolve certain of thenonferrous sulfide minerals by decomposition. The process invented forrecovery of copper from ferrous sulfides requires an oxidation steprequiring the addition of oxygen, or air, under pressure and at elevatedtemperatures. For example, Canadian Pat. No. 712,989, issued July 6,1965, to Vladimir N. Mackin and Herbert Veltman discloses a method ofleaching copper sulfate bearing pyrite ore in an aqueous acid solutionwhich contains a sufficient amount of sulfuric acid to combine with thecopper values present at elevated temperatures in the presence of freeoxygen to convert the copper sulfides into soluble copper values andelementary sulfur with little conversion of the pyrite. This leachingmethod is disadvantageous in that it l requires the addition of sulfuricacid to the leaching solution, (2) converts the sulfide sulfur from thecopper sulfide to elementary sulfur which contaminates the pyrite, and(3) for every mole of copper sulfide converted into soluble coppervalues a mole of sulfuric acid is produced and thereby increases thesulfuric acid content of the leaching solution which increases thesolubility of pyrite and other iron salts into solution to furthercontaminate the copper containing leaching solution. When treating ametal sulfide concentrate, ore and/or other concentrator sulfideproduct, the resultant production of soluble iron sulfates and excesssulfuric acid serves no useful purpose after reaching certain optimumconcentrations or iron sulfates and excess sulfuric acid in the leachingsolution. Excessive concentrations of the soluble iron salts and,sulfuric acid in the leach liquor not only dilutes and contaminates thesolution containing soluble nonferrous metal values but also increasesthe consumption of pressurized air or oxygen which is consumed in theirproduction. Pumping and pressurizing air or oxygen is an expensiveoperation, which undesirably increases the cost of producing the desiredmetals. For the recovery of copper, from the leach solution, byprecipitation, employing metallic iron as the precipitant, excessivequantities of soluble iron salts and sulfuric acid also increases theconsumption of the precipitant, metallic iron, resulting'in the furtherincreasing of the cost of producing the copper metal. British Pats. Nos.760,624 (published Nov. 7, 1956) and 801,403 (published Sept. 10, 1958)to Chemical Construction Corp. disclose other acid leaching methodsconducted at elevated temperatures in the presence of oxygen thatconvert pyrite to iron oxide, sulfide sulfur to elementary sulfur and aportion of the nonferrous mineral sulfides to soluble nonferrous metalvalues. These methods are disadvantageous in that they I) require theaddition of sulfuric acid to the leaching solution, (2) convert thepyrite to iron oxide which cannot be sold for sulfuric acid production,and (3) convert the sulfide sulfur'to elemental sulfur which occludessome of the nonferrous mineral sulfides and contaminates the iron oxide.The production of iron oxide or elementary sulfur serves no usefulpurpose in the conversion of insoluble copper sulfides to soluble coppervalues. Elemental sulfur has the tendency to form globules that occulatecopper sulfides solids which prevents conversion of the copper sulfides,and the production of iron oxide from pyrite requires a largeexpenditure of pressurized oxygen which increases production costs ofthe soluble copper values.

The present invention overcomes many of the disadvantages of the priorart leaching processes. The present method provides a method ofrecovering over percent of the water-insoluble, nonferrous metal valuesin pyrite ore with minimum (1) conversion of pyrite, (2) formation ofsulfuric acid, and 3) formation of elemental sulfur. Furthermore, thepresent method does not require the addition of acid, such as sulfuricacid, to the leaching solution or bath.

SUMMARY and, passing an oxygen-containing gas through .the slurry atabove atmospheric pressure to convert the nonferrous metal of over 90percent of the nonferrous m'etalvalues, (2) does. not requireacidification or the addition of sulfuric acid to leaching solution, (3)minimizes conversion of pyrite to soluble iron'salts, (4) minimizes theproduction of elementalsulfur, and (5) minimizes the production ofsulfuric acid and ferric'sulfate, whereby their concentration intheleaching solution is controlled. a

' BRIEF DESCRIPTION or THE DRAWING FIG. 1 is a schematic flow sheetillustrating the novel method which is'the subject matter of the presentinvention; and 7 FIG. 2 is a curve illustrating the change in copperextraction withthe change in the temperature of the leac DES CRlPTlON OFTHE PREFERRED EMBODIMENT Although the present method can be practiced ona whole I ore, such a process'would be wasteful of apparatus capacityand hence would be uneconomical. Usually, the feed material to beleached will be either a very high grade c'oncentrate or an intermediateconcentrator product of relatively high sulfur content and containingnonferrous metal values which can be economically recovered. Theconcentrator product can be a byproduct obtained by gravity separation,by sizing'separation, or by flotation separation from other processesfor other purposes. One such'byproduct which has been successfully usedas afeed material in the present process is slime tailings ofsubstantially'all 325 mesh which assays from about 0.60 percent to aboutl'.2.perce'nt copper andabout 35 percentsulpressure. The aforementioneddensity of the slurry is preferredfor both metallurgical and economicalreasons. Although some increased extractionoccurs at lower slurrydensities, lower slurry densities resultin lower plant capacity orrequire more expensive equipment and machinery to handle the increasedequivalent tonnage as the density is decreased.

The preheated slurry is then pumped into rubber-lined autoclavesoperating at an elevated pressure between 60 and l 25 p.s.i.g.,preferably at about 90 p.s.i.g. (oxygen partial pressure of 22.5 lb."sq. in.), for the oxidation leaching.

Leaching operations are carried out at a temperature 7 between about 175F. and about 250 F. As shown in FIG. 2,

fur values, such as sulfide sulfur and sulfatev sulfur.- lnsuch slimetailings the copper is .mainly' in mineral form as chalcocite, i.e.,cuperous sulfite, and is also'containedin the pyrite. v A

During the subsequent oxidation leaching there must be available anamount of sulfate sulfur, as ferric sulfate, stoichiometricallyequivalent to the amount of nonferrous metals in order to form sulfatesof the nonferrous metal values. it is also necessary to have'availableasufficient amount of sulfide sulfur as pyrite to maintainan adequateconcentration of ferric sulfate tojoxidize the nonferrous sulfides andof sulfuric acid to oxidize theferroussulfate to ferric sulfate and todissolve the nonferrous metal values; A high concentration of sulfuricacidin the leaching solution should be avoided because it reduces thesolubility of'the'soluble'nonferrous metal sulfates in the leachingliquor and increases the solubility of the iron and other contaminates.Considered from a commercial standpoint, viz the consumption of ironduring the precipitation of copper, a sulfuric acid concentration above50 grams per liter after oxidation should be avoided. In like manner,any amount of ferric sulfate above 50 grams per liter is undesirable.

It has been determined that to produce the required chemical reactions,the sulfur values content as percent sulfur of the feed material shouldbe at least 6 percent by weight, preferably in excess of 7 percent. Ifthe sulfur valuescontent of the feed material is not sufficient toproduce the desired concentrations in.the leach solution, additionalsulfur can be,

introduced in the form of elemental sulfur, pyrite sulfide sulfur,dilute sulfuric acid, or recycle solutions containing relatively highconcentrations of water-soluble sulfates such as ferrous sulfate. i

The feed material, that is the sulfur-bearing'pyrite ore, is

processed in a conventional slurry mill at from about percent to about50 percent solids, preferably about 25 to about percent solids; and isthen transferred to rubber-lined preheat tanks where it is heated to atemperature between 150 F.

and 200 F., preferably to about 180 F.,' under atmospheric theextraction starts to peak at about 175 F. and starts to decrease atabout 250? F. Approximately l80- F. is an optimum temperature because atthis temperature the chalcocite (cuperous sulfide material) isdissolved, the percentage of copper extraction is close to the maximum,and the power consumption for heating is minimized. r

In an oxidizing leach of the present type, air is the most practicaloxidizing agent. The air, and therefore the oxygen containedtherein,.can be supplied to the slurry most effectively by subsurfaceinjection of the air with mechanical dispersion of the gases in theslurry. z The quantity of oxygen is relatively critical to controllingthe amount of oxidation required for the operation, andthereforefthe-air is, metered into theheated pressurized slurry ata rateof more than about 100 standard cubic feet per minute per 1,000 cubicfeetof slurry, a rate of about 170 standard cubic feet per minutehasbeen found to give excellent'results. Because the air contains inertgases such as nitrogen, the off gas'must be bled off at approximatelythe same rate to prevent the autoclave atmosphere frombecomingdepletedof oxygen.

Somewhat lower oxygen contents can be maintained. but .the treatmenttime is correspondingly. prolonged. Although there is a correspondingincrease in theefficiency of oxygen utilization at lower oxygen contentsin the autoclave atmosphere, the necessary increase in apparatusrequirements at such lower oxygen contents offsets any savingstherefrom.

Higher oxygen partial pressures can be produced by using increasedamounts of air with the accompanying increase in power and apparatuscosts, or by increasing the bleed rate with an accompanying drop in theefficiency of oxygen utilization. Also, although higher oxygen partialpressures-would decrease the retention time'for'the chemical reactions;the

resulting increased consumption of iron and production of sulfuric acidand ferric-sulfatewould resultin higher precipitation costs, greatercontamination of the slurry or:leaching,

solution, and wasteful consumption of pyrite. Higherpressures would alsorequire more expensive autoclaves to withstand the increased pressureand more corrosive solutions.

The oxidative leaching is conducted for a sufficient period to allow theleaching of the nonferrous metal value to be substantially complete;that is, the leaching is preferably continued until at least 90 percentof the nonferrous metal values are converted into soluble values.Generally the oxidative leaching is conducted for at least a 2-hourperiod, preferably for at least a 3-hour period. Rarely does theleaching process require a period in excess of 10 hours.

Of course, some variation in the aforementioned operating conditions maybe indicated by variations in the contents of v Although slightlyadditional recoveries of nonferrous metals are achieved when highconcentrations of ferric sulfate and sulfuric acid are present in thesolution, excess amounts of ferric sulfate and sulfuric acid consume thepyrite to form soluble iron salts and elementary sulfur, and therebyreduce the amount of pyrite thereof available for sale. Ferric sulfateand sulfuric acid also consume greater quantities of precipitatingreagents (such as scrap iron) during the recovery of the nonferrousmetals from the pregnant leaching liquor, thereby increasing the cost ofthe nonferrous metal recovery.

At the end of the required residence time, the slurry is cooled,filtered, and the solid residue washed with water. The solids,containing most of the iron values, are removed from the system afterthe washing. As noted above, after proper selective leaching, much ofthe iron will remain as iron sulfide in the insoluble residue. Thesulfur content can be upgraded into a salable sulfur-bearing byproductby some ancillary process which forms no part of the preset invention.

The pregnant liquor and the wash water, containing the nonferrous metalsas dissolved sulfates, is sent to a suitable recovery system which alsoforms no part of the present inventton.

Set forth below are several examples which are illustrative I of thepresent process.

EXAMPLE 1 Samples having the following listed physical characteristicsand copper and sulfur contents were treated as follows:

' Density Assays Sample Description 7: Solids 70 Cu S No. l LeachingClassifier flow 26.8 2.47 28.02

Wet Tails Plant Classifier O flow 26.6 1.52 36.75 Composite Sample 26.71.97 32.36

The composite sample portions were aqueous slurries. The compositeslurry was transferred to a stirred autoclave, heated to 250 F. andsubjected to a pressure of 90 p.s.i.g. with air at approximately 170cubic feet per minutepcr 1,000 cubic feet of slurry. These conditionswere maintained for 3 hours. At the termination of the leaching periodthe slurry was cooled, filtered and washed with tap water. The filtratewith the included wash water and the solids residue were analyzed.

The results were as follows:

EXAMPLE 2 A composite sample of slime tailings from various disposalponds was prepared and analyzed as follows:

Sample R548 Total copper l.53% Water-soluhle copper 0.32% Total Sulfur36.84%

Total iron 35.05%

Moisture content ll.l77z

The sample was slurried with tap water from the laboratory to produce aslurry containing 27 percent solids. The slurry was transferred to alaboratory autoclave and subjected to leaching under the followingconditions:

EXAMPLE 3 A grab sample of the mill tails (copper flotation tailings)analyzed 0.43 percent copper and 43.47 percent sulfur. This material wassubjected to a pressure leach under the following test conditions:

Pulp density 3071 solids Pressure .s.i.g. Temperature 230 F. Retentiontime 5 hours Off-gas rate I70 c.f.m. per I00!) CF slur!) The results ofthis test were as follows:

Test Feed Assay Example 3 7: Cu 0.43 S 43.47 Test Residue Assay Z: 540.38 Leach Solution Assay Cu-gpl 0.44 H SOtp 2.65 Fe* -gpl 4.25 Fe -gpl2.45 Recovery Copper in solution 77.7%

EXAMPLE 4 A composite sample of slime tailings, analyzing as followstotal copper 1.15 percent and water-soluble copper 0.35 percent, wasmixed with water to produce a slurry containing 26.3 percent solids. Theslurry was transferred to the laboratory autoclave and subject toleaching under the following conditions:

Temperature l67-l 76 F. Pressure p.s.i.g. Retention time 3 hrs. Off-gasrate lSO c.f.rn. per i000 CF slurry l The results were as follows:

Test Feed Assay Example 4 Cu L159: 7: S 6.07z

Test Residue Assay 7c Cu 0.17% Leach Solution Assay Cu-gpl 3.79

Fe -gpl 13.3

Fe gpl 0.5 Recovery Copper in solution 86.0%

EXAMPLE A composite sample of slime tailings, analyzing as follows totalcopper 1.02 percent and water-soluble copper 0.34 percent, was slurriedwith an aqueous solution containing 19 grams of ferrous sulfate perliter to produce a slurry containing 26.8 percent solids and sulfurvalues, as percent sulfur, in excess of 7 percent by weight. The slurrywas transferred to a laboratory autoclave and subjected to leachingunder the following conditions:

EXAMPLE 6 A composite sample of slime tailings containing sulfur values,as percent sulfur, in excess of 30 percent by weight, analyzing asfollows total copper 1.06 percent and water-soluble copper 0.32 percentwas slurried with water to produce a slurry containing 28.2 percentsolids. The slurry was transferred to a laboratory autoclave andsubjected to leaching under the following conditions:

EXAMPLE 7 Composite samples of slime tailings containing sulfur values,as per cent sulfur, in excess of 30 percent by weight were slurried withwater to produce slurries containing 27 percent solids. The slurrieswere transferred to laboratory autoclaves and subjected to leachingunder the following conditions:

Sample A Sample B Temperature 244 F. Pressure p.s.i.g. Retention time 3hrs. Off-gas rate 200 (c.f.m. per 1000 CF slurry) The results andanalysis were as follows:

Test Feed Assay Sample A Sample 8 water-soluble Cu 010 0.095 TestResidue Assay 7: Cu 0.63 0.20 Leach Solution Assay Cu-gpl L88 3.34

Fe-gpl 9.74 6.44

Fe -gpl 0.6 05 Recovery Copper in solution 510 84.0

The above results illustrate that process temperatures around 250 F.have an effect of reducing the yield of soluble nonferrous metals, suchas copper.

EXAMPLE 8 Composite samples of nonferrous metal containing pyrite slimetailings were slurried with water to produce slimes containing 27percent solids. The slurries were transferred to laboratory autoclavesand subjected to agitated leaching under the following conditions:

Sample C Sample D Sample [1 Temperature 158l72 F. l58-174 F. 158 1701.Pressure 90 p.s.i.g. 90 p.s.i.g. 90 p.s.i.g. Retention 7 time 3 hrs. 6hrs. 5.5 hrs. Oi'fgas rate 200 200 200 (c.f.m. per 1000 CF slurry) Thecomparative results were as follows:

Test Feed Assay Sample C Sample D Sample E 7: Cu 1.06 1.17 1 l7 7:water-soluble Cu 0.095 0.13 0.22 Test Residue Assay Cu 0.20 0. 0.13Leach Solution Assay Cu-gpl 3.34 3.97 "4.41 H,SO,-gpl 1.7 Fe gpl 6.446,50 5.66 Fe -gpl 0.5 1.0 1.2

Recovery 7: Copper in solution 84.0 91.0 89.0

The above test results show that leaching or retention times that exceedthree hours under the above operating conditions do not offer commercialor economic advantages over the 3- hour leach when considering the powerexpenditure for the air supply and heat.

We claim:

1. The method of selectively recovering soluble nonferrous metalsulfates from sulfur-bearing pyrite ores with a minimal conversion ofthe pyrite and production of elemental sulfur, sulfuric acid and solubleiron sulfate values, which includes the steps of:

providing an aqueous slurry by mixing water and finely dividedsulfur-bearing pyrite ore containing nonferrous metal sulfides, theslurry containing a sufficient amount of sulfur values to form solublesulfates of the nonferrous metals;

heating the slurry to a temperature between about 175 and 250 F. andpassing an amount of oxygen-containing gas therethrough at an elevatedpressure between about 60 and 125 p.s.i.g. for a predetermined period oftime to convert the insoluble nonferrous metal sulfides intowater-soluble nonferrous metal sulfates and produce sulfuric acid andferric sulfate, said heated slurry containing sufficient amounts ofsulfate sulfur values, as ferric sulfate, to form soluble sulfates ofthe nonferrous metals, and sufficient amounts of sulfide sulfur values,as pyrite, to maintain an adequate amount of sulfate sulfur, as ferricsulfate; and

controlling the amount of oxygen-containing gas passing through theslurry so that the conversion of pyrite is minimized and both thesulfuric acid concentration and the ferric sulfate concentration of theslurry is maintained below 50 grams per liter of slurry.

2. The method defined in claim 1 in which the sulfur values content aspercent sulfur of the slurrys solids is at least 6 percent by weight.

3. The method defined in claim 1 in which the solids content oftheslurry is between about 25 and about 30 percent.

4. The method defined in claim 1 in which the temperature during theoxidative leaching step is maintained at about 180 F.

5. The method defined in claim 1 in which the oxygen-containing gas isair which is passed through the slurry during the oxidative leachingstep at a pressure ofabout 90 p.s.i.g.

6. The method defined in claim 1 in which the oxygen-containing gas isair which is injected into the slurry below the surface thereof at arate of about 170 cubic feet per minute per 1,000 cubic feet ofslurry.

7. The method defined in claim 6 in which the off gases are bled fromthe slurry at a rate of about 170 cubic feet per minute per 1,000 cubicfeet of slurry.

8. The method defined in claim 1 in which the residence time of theleach is about 3 hours.

9. The method defined in claim 1 in which following the oxidativeleaching step the slurry is cooled and filtered; the resulting filteredsolid residue is washed with water; and the resulting washings are addedto the slurry filtrate.

10. The method of selectively recovering soluble nonferrous metalsulfates from sulfur-bearing pyrite ores with a minimal conversion ofthe pyrite and production of elemental sulfur, sulfuric acid and solubleiron sulfate values, which includes the steps of:

providing an aqueous slurry by mixing water and finely dividedsulfur-bearing pyrite ore which contains nonferrous metal sulfides, theslurry containing a sufficient amount of sulfur values to form solublesulfates of the nonferrous metals and produce ferric sulfate andsulfuric acid, solids content of the slurry being from about 25 to about30 percent; and

subjecting the slurry to an oxidative leaching step for about 3 hours byheating the slurry to a temperature of from about 175 to about 250 F.and passing air through the slurry under a pressure of about p.s.i.g. sothat the nonferrous metal sulfides are converted to soluble nonferrousmetal sulfates, the air being injected into the slurry below the surfacethereof at the rate of about cubic feet per minute per 1,000 cubic feetof slurry, so that the conversion of pyrite is minimized and theconcentration of sulfuric acid and the concentration of ferric sulfateare both maintained below 50 grams per liter of slurry, said heatedslurry containing sufficient amounts of sulfate sulfur values, as ferricsulfate, to form soluble sulfates of the nonferrous metals, andsufficient amounts of sulfide sulfur values, as pyrite, to maintain anadequate amount of sulfate sulfur, as ferric sulfate.

11. The method defined in claim 10 in which the sulfur values content,as percent sulfur, of the slurrys solids is at least 6 percent byweight.

12. The method defined in claim 10 in which the off gas is removed at arate of about 170 cubic feet per minute per 1,000 cubic feet ofslurry,during the oxidative leaching step.

222g? UNITEC STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,642,435 Dated February 15, 1972 Inventofls) Eugene S. Allen and RoyceS; Gavrias It is certified that error appeers in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

- FOREIGN PATENTS OR APPLICATIONS, "51,485" (Canada) should be--519,-4s5--.

Column line 66, "association" should be --associa tions Column line 23,"or" should be -:'--of--. Column line 9, after "60" insert; --psig--.

Column Column 4, line 63, "variation" should be -uarietions--.

5, line 19, "preset" should be "present".

Column line 34, after "25" insert "7";

Column l0,line 20, after "25" insert .Column 10, line 25, after "175"!insert F.-

1 Column 10, line 41, "off gas" should be --off-gasr--.

Signed and sealed this 21st day of November 1972.

(SEAL) A-ttest:

EDWARD M.FLETcHsR,J ROBERT GOTTSICHALK alittesting Officer Commissionerof Patents J

2. The method defined in claim 1 in which the sulfur values content aspercent sulfur of the slurry''s solids is at least 6 percent by weight.3. The method defined in claim 1 in which the solids content of theslurry is between about 25 and about 30 percent.
 4. The Method definedin claim 1 in which the temperature during the oxidative leaching stepis maintained at about 180* F.
 5. The method defined in claim 1 in whichthe oxygen-containing gas is air which is passed through the slurryduring the oxidative leaching step at a pressure of about 90 p.s.i.g. 6.The method defined in claim 1 in which the oxygen-containing gas is airwhich is injected into the slurry below the surface thereof at a rate ofabout 170 cubic feet per minute per 1,000 cubic feet of slurry.
 7. Themethod defined in claim 6 in which the off gases are bled from theslurry at a rate of about 170 cubic feet per minute per 1,000 cubic feetof slurry.
 8. The method defined in claim 1 in which the residence timeof the leach is about 3 hours.
 9. The method defined in claim 1 in whichfollowing the oxidative leaching step the slurry is cooled and filtered;the resulting filtered solid residue is washed with water; and theresulting washings are added to the slurry filtrate.
 10. The method ofselectively recovering soluble nonferrous metal sulfates fromsulfur-bearing pyrite ores with a minimal conversion of the pyrite andproduction of elemental sulfur, sulfuric acid and soluble iron sulfatevalues, which includes the steps of: providing an aqueous slurry bymixing water and finely divided sulfur-bearing pyrite ore which containsnonferrous metal sulfides, the slurry containing a sufficient amount ofsulfur values to form soluble sulfates of the nonferrous metals andproduce ferric sulfate and sulfuric acid, solids content of the slurrybeing from about 25 to about 30 percent; and subjecting the slurry to anoxidative leaching step for about 3 hours by heating the slurry to atemperature of from about 175* to about 250* F. and passing air throughthe slurry under a pressure of about 90 p.s.i.g. so that the nonferrousmetal sulfides are converted to soluble nonferrous metal sulfates, theair being injected into the slurry below the surface thereof at the rateof about 170 cubic feet per minute per 1, 000 cubic feet of slurry, sothat the conversion of pyrite is minimized and the concentration ofsulfuric acid and the concentration of ferric sulfate are bothmaintained below 50 grams per liter of slurry, said heated slurrycontaining sufficient amounts of sulfate sulfur values, as ferricsulfate, to form soluble sulfates of the nonferrous metals, andsufficient amounts of sulfide sulfur values, as pyrite, to maintain anadequate amount of sulfate sulfur, as ferric sulfate.
 11. The methoddefined in claim 10 in which the sulfur values content, as percentsulfur, of the slurry''s solids is at least 6 percent by weight.
 12. Themethod defined in claim 10 in which the off gas is removed at a rate ofabout 170 cubic feet per minute per 1,000 cubic feet of slurry, duringthe oxidative leaching step.